Problem 74 - Chemistry Matter and Change

Question

Select a strong acid, and explain how you would prepare a dilute solution of the acid. Select a weak acid, and explain how you would prepare a concentrated solution of the acid.

Step-by-Step Solution

Answer

Select a strong acid, such as hydrochloric acid (HCl), and a weak acid, such as acetic acid (CH3COOH). To prepare a dilute solution of HCl, choose the desired final concentration and volume, calculate and measure the required volume of concentrated HCl, place it into a volumetric flask, and fill the remaining volume with distilled water. Gently mix the solution. To prepare a concentrated solution of acetic acid, choose the desired final concentration and volume, calculate the required mass of pure acetic acid, weigh and place it into a volumetric flask, and fill the remaining volume with distilled water. Gently mix the solution.

Step 1: Selecting a strong acid and a weak acid

To begin, we need to select a strong acid and a weak acid. A common strong acid is hydrochloric acid (HCl) and a common weak acid is acetic acid (CH3COOH, also known as vinegar).

Step 2: Preparing a dilute solution of hydrochloric acid (strong acid)

A dilute solution means the acid concentration is low in the solution. To prepare a dilute solution of hydrochloric acid, follow these steps: 1. Choose the desired concentration and volume of the final solution. For example, let's prepare 1 L of 0.1 M HCl. 2. Calculate the amount of concentrated HCl required using the dilution formula: \(C_1V_1 = C_2V_2\), where \(C_1\) is the initial concentration, \(V_1\) is the volume of the initial solution, \(C_2\) is the final concentration, and \(V_2\) is the volume of the final solution. 3. Measure the required volume of concentrated HCl using a pipette or burette. For example, if you have 12 M HCl, use the dilution formula to determine the required volume: \(12 \, \text{M} \times V_1 = 0.1 \, \text{M} \times 1 \, \text{L}\); \(V_1 = \frac{0.1 \, \text{M} \times 1 \, \text{L}}{12 \, \text{M}} = 0.00833 \, \text{L}\). 4. Pour the measured volume of concentrated HCl into a volumetric flask carefully and slowly. Make sure to wear proper lab safety equipment, such as gloves and goggles. 5. Fill the remaining volume of the volumetric flask with distilled water up to the marked line. Gently shake the flask to mix the solution. 6. Now you have prepared 1 L of 0.1 M hydrochloric acid solution.

Step 3: Preparing a concentrated solution of acetic acid (weak acid)

A concentrated solution means the acid concentration is relatively high in the solution. To prepare a concentrated solution of acetic acid, follow these steps: 1. Choose the desired concentration and volume of the final solution. For example, let's prepare 1 L of 1 M acetic acid. 2. Calculate the amount of pure acetic acid needed using the formula: \(n = M \times V = m / \text{Molar mass}\), where \(n\) is the number of moles, \(M\) is molarity, \(V\) is solution volume, and \(m\) is mass of solute. 3. Weigh the calculated mass of acetic acid using a balance. For example, acetic acid has a molar mass of 60.05 g/mol: \(n = 1 \, \text{M} \times 1 \, \text{L} = 1 \, \text{mol}\); \(m = 1 \, \text{mol} \times 60.05 \, \text{g/mol} = 60.05 \, \text{g}\). 4. Pour the weighed amount of pure acetic acid into a volumetric flask carefully. 5. Fill the remaining volume of the volumetric flask with distilled water up to the marked line. Gently shake the flask to mix the solution. 6. Now you have prepared 1 L of 1 M acetic acid solution.

Key Concepts

Dilution Formula

When preparing acid solutions, the dilution formula is a fundamental tool used to calculate the required volume of a concentrated acid needed to attain a specific concentration in a diluted form. It's based on the principle that the number of moles of acid remains constant before and after the dilution process.
This formula is expressed as:
\[\begin{equation} C_1V_1 = C_2V_2 \tag{Dilution Formula}\br\br\br\end{equation}\]Where:

• C_1 is the concentration of the starting solution (the concentrated acid)
• V_1 is the volume of the starting solution needed
• C_2 is the desired concentration of the final, diluted solution
• V_2 is the total volume of the final solution

In practice, after rearranging the formula to solve for the unknown volume V_1, you would measure this volume of the concentrated acid, then add enough solvent—usually water—up to the final desired volume V_2. This process ensures you achieve the desired molarity in a safe and precise manner. Remember to add the acid to water, not the other way around, to avoid exothermic reactions that can cause splashing.

Molarity

Molarity (M) is a measure of the concentration of a solution, expressing the number of moles of solute per liter of solution. Molarity is vital in chemical preparations as it quantifies how strong or weak a solution is, which affects the chemical reactions that it can undergo. The formula to calculate molarity is simple:
\[\begin{equation}M = \frac{n}{V}\tag{Molarity Formula}\br\br\br\end{equation}\]Where:

• n is the number of moles of solute (the substance being dissolved, such as an acid)
• V is the volume of the solution in liters

For instance, when a problem states to prepare a '1 M HCl solution,' the aim is to have 1 mole of hydrochloric acid in each liter of solution. Understanding the concept of molarity allows you to adjust concentrations accordingly to achieve the desired outcome for an experiment, be it a titration or just to prepare standard solutions for future use.

Acid Concentration

Acid concentration refers to the amount of acid present in a given volume of solution. It is a crucial concept in chemistry, particularly when dealing with reactions that involve acids. Acids come in various forms and concentrations; a 'strong' acid, like hydrochloric acid, dissociates completely in water to form free protons and its conjugate base, while a 'weak' acid, like acetic acid, only partially dissociates.
Concentrated vs. Dilute Solutions:

• A concentrated solution has a relatively high amount of solute (in this case, acid) in the solvent.
• A dilute solution has a low amount of solute compared to the solvent.

It's important to note that concentration can be altered by the process of dilution or by adding more solute to the solvent. Concentration impacts the acidity or pH of the solution, and therefore the chemical behavior of the acids. When you're mixing acids and water, always add the acid to the water to minimize the risk of splashing. Carefully managing acid concentration is crucial for ensuring safety and achieving accurate results in any laboratory setting or industrial application.